What Is An Evaporation Line on a Pregnancy Test?

If you're hoping for a positive pregnancy test, you may find yourself squinting at the test stick, willing two solid lines to appear. And if you wait long enough, both lines may indeed show up— but that doesn't necessarily mean you're pregnant. In some cases, what looks like a positive result may actually be an evaporation line (or evap line).

So what is an evaporation line? It's a marking that may materialize on a home pregnancy test after the sample has dried up and evaporated, says Robert Setton, M.D., a reproductive endocrinologist and infertility specialist at Shady Grove Fertility in New York.

An evaporation line "appears as a faint, colorless line at the location of the 'test' line," says Dr. Setton. "It's essentially an artifact of the test that only appears once the urine has dried up." Evap lines aren't an indication of pregnancy; rather, they often mean you waited too long to read the test results.

Here's how to tell the difference between a true positive pregnancy test and an evaporation line, plus tips for avoiding an evaporation line on blue dye or pink dye pregnancy tests.

How Do Home Pregnancy Tests Work?

In a successful pregnancy, the body begins producing a hormone called human chorionic gonadotropin (hCG) shortly after implantation (when the fertilized egg implants in the uterine wall). Home pregnancy tests are designed to detect hCG through a urine sample.

"hCG levels increase in early pregnancy, and many [pregnancy test] brands are able to detect hCG in the urine around the time of a missed period using what are called immunometric assay methods," explains Lisbeth Chang, M.D., OB-GYN at Dignity Health Medical Group: Northridge.

For dye-based pregnancy tests, results are interpreted by the number of colored lines that appear in the test window shortly after urinating on the test stick, says Dr. Chang. A positive result will generally show two lines while the negative test will show only one (the control line). Note that you can also use a digital pregnancy test that says the words "pregnant" or "not pregnant" instead.

At-home pregnancy tests come with very specific instructions about how and when to take them, and how and when to read the results, to avoid inaccuracy. Refer to your pregnancy test's instructions for more specific guidance.

Why Do Evaporation Lines Appear?

Evaporation lines are relatively common, and they might appear on a pregnancy test as the urine dries. They typically show up after the timeframe where you're instructed to read the results—which is generally from two to five minutes after you pee on the stick. In other words, people might see evaporation lines if they read a pregnancy test too late.

Dr. Chang adds that evaporation lines can sometimes occur due to the other chemicals in a person's urine. "As urine is drying, these chemicals can sometimes create a line on the test strip," she says.

The problem with evaporation lines is that they can be confused with a positive test result, leading a person to think they're pregnant when they're actually not, explains Dr. Chang.

How Can You Identify An Evaporation Line?

Evaporation lines are generally quite faint, and they're often colorless (or a faded gray color). You won't spot any colored dye in them. Evap lines might also appear thinner than the control line, and they sometimes don't extend fully down the track.

On the other hand, "a positive pregnancy test will appear as a colored line that should look just like the control line," says Dr. Setton. These might be blue or pink, depending on the specific test.

Still, evaporation lines can be very difficult to distinguish from faint positive pregnancy tests, which happen on rare occasions. "A faint positive test line can occur in a true pregnancy if the test is taken shortly after implantation and the hCG level is still low, or if the urine is very diluted," says Dr. Chang.

How To Avoid Evaporation Lines On Your Pregnancy Test

To avoid the disappointment (or dread) that may be caused by a false positive evaporation line, it's critical to closely follow the instructions on your pregnancy test kit. Keep in mind that not all tests are the same, so don't assume that you know how to take all pregnancy tests because you've done one before.

"It's important to read the pregnancy test in the time window recommended by the test kit," explains Dr. Chang. "Checking the test too early or especially checking it too long after the recommended window may show a false positive evaporation line."

Most home pregnancy tests need to be interpreted within two to five minutes of urination, adds Dr. Setton. "Follow the instructions on the specific at-home pregnancy test you have purchased to avoid being confused with an evaporation line."

Taking a pregnancy test first thing in the morning, when your urine is most diluted, can also ensure more accurate results. You should also avoid using expired tests, as well as those stored in very hot or cold places.

And if you're still not sure? Wait a few days and take another pregnancy test to confirm your results. Tests are more accurate if taken after the day of your missed period. "If there's ever a questionable result, reach out to your doctor to have blood work done to confirm," says Dr. Setton.